Field of the Invention
The present invention relates to a method of preventing unjust use of a personal communication terminal and, more particularly, to a method of preventing unjust use of a personal communication terminal in a communication system which is composed of portable personal communication terminals, a base station for transmitting and receiving a radio signal to and from each of the personal communication terminals and an exchange which is connected to the base station.
A personal handy phone system is a new telephone service which enables a slave phone (personal communication terminal) of a cordless digital telephone to be carried out so as to use the personal communication terminal at a given place. FIG. 10 shows the structure of a personal handy phone system. In FIG. 10, the reference numeral 1.sub.11, 1.sub.12, . . . , 1.sub.2 . . . represent portable personal communication terminals, 2.sub.1 to 2.sub.N base stations (BS) which are provided in public telephone boxes or the like so as to transmit and receive a signal to and from the personal communication terminals, and 3 an exchange which is connected to the base stations.
Each of the personal communication terminals 1.sub.11, 1.sub.12, . . . , 1.sub.2 . . . constantly compares the positional information which is supplied from the corresponding base station 2.sub.1 to 2.sub.N with the positional information which it holds, and if it is not the same, the corresponding personal communication terminal stores the newly received positional information and transmits the terminal ID to the exchange 3 through the base station so as to register the position. At the time of calling, if the user of the personal communication terminal 1.sub.11 inputs the telephone number of a called party, the base station 2.sub.1 designates the communication channel and connects the personal communication terminal 1.sub.11 with the exchange 3. Thereafter, the exchange 3 executes the call setting process between the exchange 3 and the personal communication terminal 1.sub.11 through the base station 2.sub.1, and connects the personal communication terminal 1.sub.11 with the terminal of the called party. At the time of reception of call, the exchange 3 obtains a base station in a zone to which the terminal of the called party belongs on the basis of the telephone number of the called party and the registered positional information which is stored by a position registering process, and informs the base station of the reception of the call. On the basis of the information about the reception of the call, the base station calls the terminal of the called party, designates the communication channel, and connects the exchange 3 with the personal communication terminal.
It is expected that such a personal handy phone system will come into wide use and various kinds of terminals will be developed in the future. It is then necessary to guard against the unjust use of a terminal. For this purpose, it is necessary to incorporate an unjust use checking mechanism, namely, an "identification mechanism" into a newly developed terminal.
In a conventional personal handy phone system, the exchange 3 is provided with a data base for "cryptographic keys" and ascertains whether or not the corresponding terminal is justly used by starting a identification process with respect to the terminal at the time of calling, reception and position registration.
FIG. 11 is an explanatory view of the identification process at the time of calling. As shown in FIG. 10, identification types 4a and cryptographic keys 4b are registered in the memory 3a of the exchange 3 in correspondence with the terminal IDs of the service subscribers. The terminal ID 5a and the cryptographic key 5b are registered in the ROM of each personal communication terminal.
(1) When the personal communication terminal 1.sub.11 calls by inputting the phone number of a called party, it transmits a SET UP message to the exchange 3 through the base station 2.sub.1. (2) The exchange 3 obtains the identification type 4a and the cryptographic key 4b corresponding to the calling terminal ID which are included in the SET UP message, and creates random numbers on the basis of the identification type 4a and the cryptographic key 4b. (3) The exchange 3 transmits the random numbers to the personal communication terminal 1.sub.11. (4) When the personal communication terminal 1.sub.11 receives the random numbers, it enciphers the random numbers by using the cryptographic key 5b it holds, (5) and transmits the cryptogram to the exchange 3. (6) In parallel with this operation, the exchange 3 enciphers the random numbers by using the cryptographic key 4b, and (7) compares the cryptogram obtained with the cryptogram received from the personal communication terminal 1.sub.11. (8) If the cryptograms are not coincident, the exchange 3 regards the call as a call from a noncontractor and rejects communication, (9) while if the cryptograms are coincident, the exchange 3 transmits call reception approval to the personal communication terminal 1.sub.11, and continues the calling processing so as to connect the personal communication terminal 1.sub.11 with the terminal of the called party.
By starting this identification process, it is possible to prevent the unjust use of the service by a noncontractor. However, if the contents of the ROM of a personal communication terminal of a real contractor are copied by a mala fide third party and the copied ROM is mounted on a personal communication terminal which is not contracted, it is possible to freely call from the unjust terminal. In this case, communication from the unjust terminal is freely possible and the owner of the personal communication terminal of the original ROM (real contractor) is naturally charged for the communication until the owner of the personal communication terminal perceives the existence of an unjust user of the terminal from the bill charging him too much for the actual calls and takes a measure such as a change in the cryptographic key.
Such a problem of unjust use is also encountered in a mobile communication system such as a car telephone system and a portable telephone system. To solve this problem, the following method of preventing the unjust use of a mobile communication system is proposed (U.S. Ser. No. 08/398,217, application date Mar. 2, 1995), the mobile communication system being composed of a mobile network which includes a mobile communication exchange office, a plurality of base control stations connected to the mobile communication exchange office and a plurality of base stations controlled by the corresponding base control stations, and a plurality of mobile stations each of which exists in a radio zone of the predetermined base station so as to transmit and receive control data for calling processing and a communication signal to and from the corresponding base station through a control channel and a communication channel, respectively.
The mobile network counts the number of calls from a mobile station in a predetermined time of period, stores the number in correspondence with the ID of the mobile station, and periodically reports the number of received calls to the respective mobile stations. Each of the mobile station counts and stores the number of calls which have been requested in a predetermined period of time, and when the number of received calls is reported from the mobile network, the mobile station compares the number of received calls with the stored number of calls which have been requested and reports the agreement/disagreement to the mobile network. If the mobile network receives the report of disagreement, it thereafter stops receiving a call from the corresponding mobile station. In this manner, it is possible to prevent the unjust use of a mobile terminal provided in the mobile station even if the contents of the ROM of the mobile terminal of a real contractor are copied by a mala fide third party. However, this method of preventing unjust use under the control of the mobile network has the following problems.
(1) Each of the mobile stations is required to have a function of identifying the number of received calls which is sent from the mobile network, a function of comparing the number of received calls with the number of calls which have been requested, and a function of reporting the agreement/disagreement to the mobile network. Therefore, each mobile station has a complicated structure and becomes expensive.
Personal communication is a communication system which attaches much importance to a small size, a light weight and a low cost of a personal communication terminal. It is therefore necessary to provide as little service function as possible for a personal communication terminal. However, the proposed method of preventing unjust use requires the above-described functions, which makes the structure complicated and raises the cost.
(2) In the proposed method of preventing the unjust use of a mobile terminal, when the mobile network finds an unjust mobile station having a predetermined ID, it thereafter stops receiving the request for call from the mobile station having the ID. According to this method, a procedure for restoring the calling function to the real mobile station is complicated and calling is impossible for a long time during the procedure.
(3) In the proposed method of preventing the unjust use of a mobile terminal, since the mobile network periodically reports the number of received calls, the number of received calls is also open to an unjust mobile terminal. There is therefore a risk of the unjust use of the open information.
(4) Since the mobile network must periodically report the number of received calls to each of a multiplicity of mobile stations and execute predetermined processing when the agreement/disagreement is reported, the load of the mobile network increases.